Acetic acid is commercially produced by methanol carbonylation. Prior to 1970, acetic acid was made using a cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem associated with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction (CO+H2OCO2+H2). Water and hydrogen are needed to react with precipitated Rh(III) and inactive [Rh4(CO)2] to regenerate the active Rh(I) catalyst. This large amount of water increases the amount of hydrogen iodide, which is highly corrosive and leads to engineering problems. Further, removing a large amount of water from the acetic acid product is costly.
In the late '70s, Celanese modified the carbonylation process by adding lithium iodide salt to the carbonylation. Lithium iodide salt increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide salt promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the early '90s, Millennium Petrochemicals developed a new rhodium carbonylation catalyst system that does not use iodide salt. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Millennium catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One important issue in the low-water carbonylation process is to measure and control the concentration of carbon monoxide in the reactor liquid so that a sufficient amount of hydrogen is generated to allow the reduction of the Rh(III) to active Rh(I) catalyst. Direct measurement of the carbon monoxide concentration in the reactor liquid is a challenge and no direct analytical method has been developed in the art. U.S. Pat. No. 7,476,761 teaches an indirect measurement. According to the '761 patent, the reactor liquid is withdrawn from the reactor and flashed into a gas mixture and a liquid. The gas mixture contains carbon monoxide and other volatile components. The gas mixture passes through a control apparatus where the carbon monoxide is measured. The carbon monoxide concentration in the reactor liquid is then estimated or calculated based on the carbon monoxide concentration in the gas mixture.
U.S. Pat. No. 6,552,221 also teaches process control for acetic acid manufacture. According to the '221 patent, samples are collected from columns and/or transfer lines downstream of a reactor vessel, and the concentration of one or more components in the sample is measured by an infrared analyzer. The concentration measurements are then used to make adjustments in the concentration of components in the reaction system, directly or indirectly, such as by adjusting the temperature profile in a particular column, the flow rate of solution into or out of a column, the vent gas rate out of the reactor or a column, or the addition or extraction of a component to or from the solution. The components measured include water, acetic acid, methyl acetate, methyl iodide, aldehydes, hydrocarbons, propionic acid, and hydrogen iodide. Similarly, U.S. Pat. No. 6,362,366 teaches an online method to measure components in the reactor mixture.
New methods for measuring carbon monoxide and other components in the reactor liquid of the methanol carbonylation are needed. Ideally, the method can directly measure the carbon monoxide concentration in the reactor liquid.